Polygon area

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef vector<ll> vl;
typedef pair<ll,ll> pl;

typedef long long C;
typedef complex<C> P;
#define X real()
#define Y imag()

#define F first
#define S second
#define PB push_back
#define MP make_pair

#define REP(i,a,b) for (ll i = a; i < b; i++)



int main() {
    ll n;
    cin >> n;

    pair<ll,ll> v[n+1];
    ll x,y;
    REP(i,0,n) {
        cin >> x >> y;
        v[i] = {x,y};
    }

    v[n] = v[0];

    // REP(i,0,n+1) {
    //     cout << v[i].first << " " << v[i].second << endl;
    // }

    ll sum = 0;
    REP(i,0,n) {
        sum += v[i].first * v[i+1].second - v[i+1].first * v[i].second;
    }


    if(sum <0) sum *= -1;

    cout << sum << endl;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED